This invention relates to telecommunications systems, and more particularly to communication systems for mobile terminals.
A cellular mobile system consists of several building blocks, the most basic of which include the mobile station, the base station, and the mobile services switching center. The functions of each of these are well known in the art. The mobile station is used by mobile stations to communicate with the cellular system. The types of mobile stations that exist vary widely in range from, for example, hand-held telephones, to laptop computers or other personal communications devices. A mobile station communicates with the cellular system using a radio channel to a base station. Base stations are responsible for communication over the air to and from the mobile stations within its geographic area of assignment. The base station communicates with the mobile services switching center (MSC) which is responsible for all switching functions related to call processing. The MSC communicates with the base stations on one side and with external core networks on the other side.
The cellular network is divided geographically into a plurality of cells defining graphic areas where radio coverage is available. Each cell is serviced by one base station and employs one or several frequencies (depending on traffic load) which are different from the frequencies employed by neighboring cells.
FIG. 1 illustrates an example embodiment of a cell design for the mobile communications system. Each of the circles shown in FIG. 1 is a coverage area for a particular base station, with the base station shown as a dot in the center of the circle. Thus, cells A1, A2, A3, and A4 are geographical coverage areas for the mobile stations moving through them. Respective cells A1, A2, A3 and A4 are geographically defined by the effective air communication distance which the base station associated with the cell can provide. The distance is dependent upon a number of factors, such as the power level of the signal output of the base station and of the mobile stations within the base station cell. The base stations A1xe2x80x2, A2xe2x80x2, A3xe2x80x2, and A4xe2x80x2 are shown within their respective cells A1, A2, A3, and A4.
Traditionally, as a mobile station moves within a particular geography, it may move from one geographical cell area to another geographical cell area thus result in a hand-off procedure between the respective base stations of the cells. Thus, for example, when a mobile station is in cell A1 and is engaged in an active call, it is communicating with base station A1xe2x80x2. But, when the mobile station moves from the cell A1 into, for example, cell A2, the base station A1xe2x80x2 will hand-off service for the mobile station to the new base station A2xe2x80x2. Ideally, the hand-off procedure is seamless to the user of the mobile station. There are a number of known hand-off techniques which can be employed to improve the mobility and seamlessness of the mobile stations during hand-off procedures.
As one of ordinary skill in the art will understand, each of the cells A1, A2, A3, and A4 have a inherent limitation on the amount of traffic that they can process. Thus, each cell has an associated bandwidth with which it must (ideally) accommodate all of the mobile stations within its geographical area. Once the bandwidth is employed by active mobile stations, no further traffic service can be provided by the base station in a particular cell until one of the active mobile stations concludes its call and thereby releases some bandwidth for other mobile stations. Availability of sufficient bandwidth in respective cells becomes increasingly important as more and more mobile stations are being used within the cellular networks. As a result, the geography of the cell structures (i.e., the amount of coverage area that each particular cell services) is a design factor in creating cellular networks. Thus, the uniformly sized and shaped cells shown in FIG. 1 may not be the norm, since network designers will typically reduce the size of cells in high traffic areas and increase the size of cells in low traffic areas. Still, new and improved techniques for efficiently using the bandwidth available in a cellular network are commercially valuable advances.
In accordance with the preferred embodiment of the present invention, a geographical area is divided geographically into overlapping layers of differently sized cells. That is, a first layer of cells divides a geographical area into small cell groups and then the same geographical area is divided again into larger, overlapping, cell groups. Still further overlapping layers may be employed in addition to the first two layers. This is generally referred to as a hierarchical cell structure.
In the hierarchical cell structure, the lower layer of cells (the geographically small cells) offer higher bandwidth to the mobile stations within it, while the higher layer cells (the geographically larger cells) offer lower bandwidth availability to the mobile stations in its geography, but provide coverage over a larger geographical area.
Thus, in this hierarchical cell structure, geographically small but high bandwidth-available cells are located within geographically larger cells of less bandwidth-availability.
In the preferred embodiment of the present invention, the bandwidth requirements of a mobile station are taken into account when assigning a cell to the mobile station following a call request. A mobile station sends a bandwidth requirement request with a call request, then the bandwidth request is used to determine whether the mobile station should be assigned to a cell having smaller geography and larger bandwidth availability or to a cell having larger geography and smaller bandwidth availability.